Typically, the fuel control serves to provide optimum engine operation by open loop scheduling of fuel flow as a function of compressor speed and compressor pressure for steady state operation and compressor speed, compressor inlet temperature/or compressor inlet pressure and compressor discharge pressure for acceleration limiting. For example such a control is exemplified by the JFC-60 manufactured by the Hamilton Standard Division of United Technologies Corporation and U.S. Pat. No. 2,822,666 granted to S. G. Best on Feb. 11, 1958 assigned to the same assignee. In these controls fuel flow is scheduled by providing a W.sub.f /P.sub.3 signal as a function of the above mentioned parameters and multiplying that signal by actual P.sub.3, where W.sub.f is fuel flow in pounds per hour and P.sub.3 is compressor discharge pressure in psi to obtain proper fuel flow. Because of the advent of more complex engines and their cycles, the inclusion of variable engine geometry such as variable vanes, exhaust nozzles and the like; a supervisory control which may be electronic, monitors and computes these engine variables to coordinate them and/or assess engine performance so as to readjust fuel flow and/or manipulate these engine variables so as to achieve a more optimum engine performance for the entire flight envelope. Such a supervisory control is disclosed in U.S. Pat. No. 3,797,233 issued to William L. Webb et al on May 19, 1974 and also assigned to the same assignee, and incorporated herein by reference.
Notwithstanding the above, such heretofore known controls inherently have certain deficiencies occasioned merely by having open loop scheduling. The schedule is primarily predicated on a particular engine characteristic and is designed to provide optimum performance initially. It does not, however, account for errors and deficiencies that arise out of the engine aging. Additionally, errors and deficiencies occur as a result of bleeding compressor air, extracting horsepower, inaccurate sensors, pressure distortions and the like.
I have found that I can obtain an improved thrust performance of say 10-20% gain during transonic aircraft flight modes by closing the loop on fan pressure ratio which inherently obviates these errors and deficiencies during this regime. In a turbofan installation this invention contemplates readjusting fuel flow to the gas generator to operate at scheduled fan pressure ratio. Where the installation includes variable exhaust nozzles the area thereof can be manipulated in lieu of the fuel flow adjustments.
In either event, by holding fan pressure ratio within a 2% tolerance of its schedule the following advantages can be realized.
1. 8-10% pressure ratio uncertainty due to .+-.6.sup.o T.sub.T2 error is eliminated (.+-.6.sup.o T.sub.T2 schedules N.sub.2 too high which results in 8-10% high fan pressure ratio).
2. High spool variable vane error is compensated because primary fuel flow will adjust to maintain fan turbine work.
3. Deterioration of core engine will be compensated because the primary fuel flow will adjust to maintain fan turbine work.
4. Fan vane error effects on surge line will be compensated somewhat because surge pressure ratio will maintain operating pressure ratio as f(N.sub.1 C.sub.2) direction of corrected N.sub.1 speed. (current control will match up fan pressure ratio with cambering)
5. P.sub.T2 probe can be located in average low pressure area so that during maneuver when low pressure exists, the control will automatically reduce fan pressure ratio.